1.- A catalog that registers all the days of a year, distributed in weeks and months, with astronomical data, such as time of sunrise and sunset, the moon phases, or with religious information such as patron saints and festivities.

2.- A time division system, all of the world’s cultures have their calendars initially lunar and afterwards lunar-solar. The Chaldeans and Babylonians passed their calendric knowledge to the Egyptians, these in turn to the Greeks and these finally to the Romans who adopted it for their common use.

From the beginning of civilization there has been a very close link between astrology and the development of the calendar. The importance of this connection is evident considering the need to determine the times for the most basic functions of early societies such as agriculture and the celebration of religious events.

The most ancient calendars were probably based on lunar observation since the Moon’s phases take place in an easily observed interval. It is most likely that the sighting of the crescent Moon marked a new time period. It was observed that recurrent Moon’s phases were about 29 days apart. This gave birth to the first lunar calendars containing 29-30 days per time period (month), but since the sum of twelve or thirteen months differ from the length of a tropical year this calendar was not completely suitable for agricultural practices.

Due to this difference and in order to keep in step with the Sun, the lunar-solar calendars were born, adding a complementary time period to the total of days in the Moon’s cycles so as to equal the solar year. Many of these calendars, with variations, existed through time in different areas of the world. In pre-Columbian America the Maya and Aztec calendars were very important. They are remarkably accurate and are made of 18 months of 20 days plus five supplemental days.

Later the solar calendars came to be, for example the Julian calendar which was instituted in Rome by Julius Caesar in 46 BC. This calendar set the year’s length at 365 days and added one day to the year every four years. Pope Gregory XIII modified this calendar in 1582. And even though the Gregorian calendar is a solar calendar in the sense that it does not take into consideration the Moon in its calculations, it does contain rules for determining Easter and other religious holidays which are based on both the Sun and the Moon. The Gregorian calendar is used today in most of the world, it is divided into the twelve months we all know.

Historically people have sensed the need to have a fixed point to start their time calculations. In order to do this generally the starting point has been determined either by a historical event (the birth of Jesus) or by a hypothetical event (the date of the world’s creation). Of all known cultures the Maya seem to have been the first to discover the need for such a date, using probably an astronomically significant or a hypothetical event they placed at 3114 BC.